Downregulation of Dyskerin Impairs Expansion of Normal Hematopoietic Progenitors and Erythroid Colony Formation in the Absence of Apparent Telomere Shortening

Dyskerin is an RNA binding protein that functions as a core component of the telomerase holoenzyme as well as in ribonuclear protein complexes involved in ribosome biogenesis and RNA processing. Within the telomerase holoenzyme, dyskerin stabilizes the telomerase RNA component (hTR) that functions as a template for synthesis of telomeric DNA at chromosome ends. Telomerase is active in malignant cells as well as normal hematopoietic stem and progenitor cells where it enables long-term cell replication.

Mutation of the gene encoding dyskerin (DKC1) is the underlying cause of the inherited disease dyskeratosis congenita (DC), which is characterized by low levels of telomerase activity and shortened telomeres. Bone marrow failure is the most common cause of mortality in DC, however since dyskerin has telomerase-independent roles in fundamental cellular processes, it is not clear whether telomerase insufficiency is the sole cause of hematopoietic dysfunction in this disorder.

To gain further insight to the consequences of deregulated expression of dyskerin in haematopoietic cells, we transduced normal human cord blood CD34+ progenitor cells with retroviral vectors encoding two different shRNAs targeting dyskerin. Transduced cells from seven independent experiments were FACS sorted then assayed for expansion and differentiation in vitro, clonogenic potential, telomerase activity and telomere length maintenance. Results from the functional assays showed that depletion of dyskerin impaired cell expansion and compromised the formation of BFU-E colonies. In contrast to BFU-E, CFU-GM colonies did not appear to be altered by dyskerin depletion. Analysis of non-transuduced cells subject to ex vivo expansion along erythroid and myelomonocytic lineages revealed that erythroid precursor cells expressed disproportionately high levels of dyskerin, and correspondingly high telomerase enzyme activity. shRNA-mediated downregulation of dyskerin lowered hTR levels and quenched telomerase activity in the erythroid precursors without altering expression of the telomerase catalytic component, hTERT. These results suggest that upregulation of dyskerin bolsters telomerase activity and renders erythroid precursors vulnerable to dyskerin depletion. However in spite of the reduction in telomerase activity induced by dyskerin shRNA, Southern blot analysis showed that the rate of telomere length shortening was not significantly altered by dyskerin depletion over the time course of these experiments. Together the data show that dyskerin is upregulated during erythroid differentiation, and has an essential function in the erythroid lineage that appears to be independent of its role in telomere length maintenance.